Commonly assigned U.S. patent application Ser. No. 07/725,142 by Kwok C. Tam discloses method and means for accomplishing region of interest imaging Of a portion of an object irradiated in a field of view of a cone beam source. The region of interest may be a select portion of an object wherein the object is wholly engulfed within the field of view of a cone beam source. Alternatively, the region of interest may be only that portion of an object which fits within a field of view of a cone beam source when the entire object is too large to be wholly irradiated thereby. In either case, this region of interest is rotationally scanned by a cone beam irradiating source at its upper and lower extent along two scanning paths which serve to bound the region of interest. To ensure that a complete Radon data set is acquired for exact image reconstruction, the upper and lower scan paths are connected by a scan path therebetween in order to provide a complete scanning trajectory. Cone beam attenuation data are acquired by a suitable surface array radiation detector wherein the source and array detector are mutually fixed with respect to one another so as to rotatably scan the region of interest in order to acquire cone beam attenuation data at the detector surface for a plurality of source positions along the scanning trajectory.
To ensure exact image reconstruction, cone beam attenuation data must be acquired in a manner which fills Radon space over a so called `region of support` in Radon space corresponding to the field of view occupied by the region of interest of the object in real space. Such filling provides sufficient Radon data to completely and exactly reconstruct a 3DCT image by a process of inverse Radon transformation. In so doing, at least a requisite core number of necessary data points in Radon space is selectively retained, these data points contribute to imaging the region of interest in Radon space. A 3DCT cone beam reconstructed image obtained by inverse Radon transformation utilizes a mathematical point by point inversion technique. The Radon inversion technique is inherently a computationally intensive process which becomes unduly burdened by tracking those Radon data points which either do not contribute or redundantly contribute to reconstruction of a 3D image of the region of interest. Typically, either all collected data throughout Radon space is indiscriminantly retained for point by point inversion processing, or a truncated subset of Radon data representing only source beams which actually pass through the object are selectively retained for point by point inversion processing. Truncation boundaries in Radon space are typically identified by the use of projection and/or intersection operations which are easier to apply than direct point by point mathematical manipulations.
In a typical 3DCT reconstruction by Radon inversion, planar integrals corresponding to beam attenuation detector line integrals are calculated and organized as discrete data points in Radon space. Radon data points are organized onto an arbitrary set of planes in Radon space, wherein each surface of integration is used to calculate a Radon derivative corresponding to a single data point in Radon space. These discretely organized Radon data points are typically partitioned and selectively retained or discarded according to whether or not corresponding surfaces of integration intersect the region of interest of the object. By its mathematical nature, Radon space is a collection of discrete Radon data points each corresponding to a surface of integration, e.g. a planar integral. For each integration plane that intersects the region of interest, the corresponding computation of a Radon derivative, i.e. a Radon data point, depends upon the manner in which that plane intersects with the region of interest. Thus, the adequacy of filling the region of support in Radon space is generally assessed by first partitioning those integration planes which contribute to data points in Radon space as follows:
1. An integration plane that does not intersect the region of interest;
2. An integration plane that intersects the region of interest only;
3. An integration plane that intersects the region of interest and also either the region above or the region below, but not both;
4. An integration plane that intersects the region of interest and also both the region above and the region below.
For case 1, the planar integral will always be zero, thus no Radon derivative need be computed;
For case 2, the Radon derivative is computed in a standard manner requiring no further consideration as the Radon data are not corrupted by any contributions other than those due to the region of interest itself.
For case 3, the planar integral is computed from cone beam data according to copending patent application Ser. No. 07/725,142 wherein cone beam data originating outside the region of interest are set to zero before computation of the Radon derivative. This eliminates any additive contributions that would otherwise corrupt the data set.
Case 4 is the most general situation to be addressed as it encompasses cases 2 and 3. For case 4, such zeroing does not suffice, as corrupting contributions do not simply additively contribute but collectively cooperate with contributions from other source positions to provide unwanted corruption of the resulting Radon data set. In such case, the Radon derivative is obtained by adding the results computed from cone beam data derived from more than one source position.
The procedure is illustrated in FIG. 1 showing a typical integration plane 1 of the case 4 category. Plane 1, herein the plane of the figure, intersects source position 2 identified by an "A" on an upper scan path at level 3 and source position 4 identified by a "B" on lower scan path at level 5. That portion of plane 1 intersecting region of interest 14 of object 22 is divided into two partial planes by line 6 which connects points A and B to provide a common boundary therebetween. The Radon derivative for the upper portion, indicated by partial plane 8, is computed from cone beam rays emitted at source position A within an angular range defined between an upper boundary at level 3 and line 6. Similarly, the Radon derivative for the lower portion, indicated by partial plane 12, is computed from cone beam rays emitted at source position B within an angular range defined between the lower boundary at level 5 and line 6. Thus, unwanted contributions due to those portions of object 22 outside region of interest 14, i.e. beyond an upper scan path at level 3 and a lower scan path at level 5, which corrupt computation of planar integrals, and thereby Radon derivatives, can be eliminated by discarding all cone beam data whose paths traverse the region beyond that bounded by upper scan path at level 3 and lower scan path at level 5. Those source beams that penetrate some remainder of the object as well as region of interest 14, provide cone beam data that is not attributable solely to the region of interest i.e. the Radon data set is corrupted. Manipulation of Radon data by first partitioning integration planes into the cases previously discussed has typically been a necessary prerequisite for eliminating these unwanted corrupting contributions. Such processing has been required to ensure that an uncorrupted yet sufficiently complete Radon data set is acquired for reconstructing of an exact 3D image of the region of interest.
Typically, truncation of unnecessary data has taken place only in Radon space, in order to assure the availability of sufficient Radon data points for processing an exact image of the region of interest. Performing truncation at this stage in the processing of a reconstructed image by Radon inversion needlessly wastes time, computer resources, and money. It is therefore desirable to reduce the requisite number of computational operations at the earliest possible opportunity in order to expedite image processing without requiring any of the prior categorization and manipulation of integration planes.